Hidden and inaccessible corrosion of metal structures is an unavoidable problem, in spite of all contemporary technological protection means, since almost all environments are corrosive to a certain degree. In particular, moisture in the air along with its atmospheric content (acids, salts and organic compounds) can penetrate within almost any metallic structure and cause corrosion.
When corrosion occurs on outward surfaces, it may be observed by visual checking. More difficult is the detection of corrosion inside structures, when it is hidden between different structure elements. In order to verify the existence of hidden corrosion inside of such structures it is necessary to disassemble the entire structure, an action that is very expensive and sometime practically impossible.
The problem of early detection of hidden corrosion is crucial in maintenance of aging aircraft Conventional non-destructive inspection (NDI) techniques such as X-ray radiography, eddy current, ultrasonic, acoustic emission, etc. are used to solve this problem but are not always reliable in detecting hidden and inaccessible corrosion.
U.S. Pat. No. 5,549,803 describes a smart fastener laving corrosion detection features for detecting hidden corrosion in aircraft structures. Within the fastener there is a corrosion array which receives an electrolyte that corrodes sacrificial materials formed on the electrodes of the corrosion detecting-array. The information sensed by the corrosion sensing electrodes of the array is in the form of very small electrical signals that are amplified and stored in an analog storage memory. An interface circuit enables a user to electronically access corrosion and related signals without removing or in any way dismantling the fastener.
U.S. Pat. No. 5,446,369 describes a corrosion monitor system to facilitate the detection and monitoring of material corrosion in remote areas. Sensors, which react to a corrosive environment similar to the material to be monitored, are placed in isolated areas and connected to a central control system via shielded cables. The system operates on the principle that corrosion of a metallic conductor will cause a corresponding increase in the cross-sectional electrical resistance of that conductor. This change in resistance can be detected and monitored by passing a known constant current through the conductor and comparing the voltage across the conductor with that of a controlled reference conductor which is not exposed to the corrosive environment.
Truong et al. developed a sensor for water detection in aircraft adhesive bond lines by embedding a compressed polypyrrole powder in an epoxy adhesive sandwiched between two aluminum substrates. An interaction between polypyrrole and water resulted in a rapid increase in both DC and AC resistivity.